Summary of Significant Accounting Policies - Cash and Cash Equivalents and Foreign Currency Gains (Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Abstract]
Short-term restricted cash	$ 0.4	$ 0.8
Foreign Currency Gains and Losses
Total foreign currency transaction gains (losses)	$ (1.6)	$ (3.3)	$ (2.1)